1. Field of the Invention
The present invention relates to a radiation image conversion sheet, and more particularly to a gradient radiation image conversion sheet.
2. Prior Art
The radiation image conversion sheet (hereinafter referred to simply as "image conversion sheet") has hitherto been used in various fields of art including a medical field, where it is used for radiography for medical diagnosis or other medical purposes such as direct radiography, photofluorography or fluoroscopy, and industrial applications where it is used for non-destructive inspection of various materials, while contacting close to an X-ray photograph film (hereinafter referred to simply as "film") to improve the sensitivity of the photographing system. Alternatively, the X-ray is converted into a visible ray thereby and then the thus converted visible ray is photographed on a film. Principally, such an image conversion sheet includes a support, such as paper or plastic materials, and a fluorescent layer disposed on one side thereof. The fluorescent layer is composed of a phosphor for emitting high luminance light upon irradiation with a radiation, and a binder resin in which the phosphor is dispersed. In general, the surface of the fluorescent layer (i.e. the surface opposed to the side on which the support is applied) is protected by a transparent protective layer made of, for example, cellulose acetate, cellulose acetate butylate, polymethyl methacrylate or polyethylene terephthalate. There has also been known in the art an image conversion sheet in which a light reflecting layer or a light absorbing layer is disposed between the support and the fluorecent layer. In radiographic operation, the image conversion sheet is placed closely over the film so that the fluorescent layer faces to the film through the transparent protective layer. It is a common practice that two image conversion sheets are contacted closely on both side faces of the film, and the film sandwitched by the two image conversion sheets is held in a frame, a so-called "cassette", ready for photographing.
The method using the combination of an image conversion sheet and a radiophographic film having an emulsion layer composed of a silver salt photosensitive material for forming a radiographic image is referred to as radiophotography. In recent years, there is a demand for a method capable of forming a radiographical image without using a silver salt, in view of the shortage of silver resource.
As one proposal of the radiation image conversion method wherein no silver salt is used, a radiation image conversion panel comprising a stimulable phosphor has been proposed by U.S. Pat. No. 3,859,527. In this connection, the stimulable phosphor means a phosphor which emits light upon irradiation with a radiotion ray followed by irradiation with an electromagnetic wave selected from visible and infrared rays, the radiotions including electromagnetic wave or particle ray such as X-ray, alpha-ray, beta-ray, gamma-ray, high energy neutron ray, electron ray, vacuum ultraviolet ray or ultraviolet ray. In this known method, the radiotion ray passed through the object to be photographed is absorbed by a stimulable phosphor in the panel, and then the panel is scanned with an electromagnetic wave (acting as a stimulating ray) selected from visible and infrared rays to pick up the radiographical image as a time sequence of stimulated lights which are electrically processed to form an image.
On the other hand, in diagnosis of thoracic diseases, such as lungs cancer, by the use of chest X-ray radiography, it becomes necessary increasingly to observe the trachea and bronchi overlapping or interlocked complicately with each other in the thoracic vertebra and heart in addition to the observation of the photograph of the lung field for correct and precise diagnosis. However, an ordinary image conversion sheet generally has an even sensitivity throughout the overall area thereof so that the fluorescent light intensity of the light emitted from the fluorescent layer irradiated with X-ray is substantially constant over the entire area, and the film has a constant sensitivity throughout the entire surface area thereof. When the chest of a patient is observed through a simple radiography or a tomography, it becomes difficult to inspect or observe the lung field and simultaneously the interlocked trachea and bronchus in the thoracic vertebra and heart from single sheet of X-ray photograph. In detail, various organs in the chest have individual transmission factors to X-ray which are different from each other in a wide range, so that the film densities of the thoracic vertebra, heart and lung field become denser in this order to distribute in a relatively wide range. Accordingly, when photography is conducted under an X-ray exposure condition optimum for obtaining the film density suited for observation of the lung field, the film densities of the thoracic vertebra and heart becomes too low due to absorption of X-ray by these organs, leading to the result that the trachea and bronchus overlapped with these organs are not photographed to make it impossible to inspect the trachea and bronchus. Conversely, when radiography is conducted under an X-ray exposure condition for obtaining a film density suited for inspection of the trachea and bronchus, the density of the lung field image becomes excessively high for inspection of the lung field.
Under these circumstances, it has been a common practice to photograph plural X-ray photographs under X-ray exposure conditions suited respectively for obtaining photographs of the lung field and of the trachea and bronchus to have the images thereof having proper densities, or the X-ray photograph of the chest has been inspected together with the result of bronchography conduced by the use of a contrast medium to make a diagnosis of the diseases in the chest. However, it is not preferred to have plural X-ray photographs since the exposure dose to the patient is increased, and the X-ray bronchography suffers the patient a severe pain and has additional problems that it cost times and expenses amounting to several times as much as those required for a simple photography.
In order to solve the aforementioned problems, proposals have been made to use an image conversion sheet, i.e. a gradient image conversion sheet, with a speed partially varied. The gradient image conversion sheets of preceding proposals include, a type wherein the thickness of the fluorescent layer is partially varied to effect gradation in speed or sensitivity, a type wherein a light reflecting layer containing a white pigment or the like is dispersed at localized portions between the support and the fluorescent layer to improve the sensitivity of the portion provided with the light reflecting layer to achieve the aimed gradation, a type using phosphors having different luminance so that the speed is complemented, and a type wherein a light absorbing layer composed of a coloring agent having a body color, such as black, blue or red, is introduced at desired portion of the interface between the support and the fluorescent layer to lower the speed of the portion provided with such a coloring agent.
However, the gradient image conversion sheets of the known types tend to give clear border lines between the high speed portions and the low speed portions due to the caused inherent to the technology for preparing the same, and thus there appear border or marginal areas of the gradient region on the the X-ray photograph boarder regions in which the photographing speeds are abruptly changed to provide adverse influences on diagnosis. The known type gradient image conversion sheets have further disadvantages that it is hard to produce the products satisfying the desired requirements or specifications, and that it is not easy to produce products of uniform quality by mass production.
As another proposal for solving the aforementioned problems, Japanese Utility Model Publication No. 19425/1980 discloses a system in which the speed or sensitivity is varied by the use of a printed card board. However, even if the card board is printed with a gradation printing of ordinary type, there appears a clear image of the border between the portion bearing the printing of lightest shade and the unprinted portion to provide inconvenience for clinical application. A still further proposal has been made by Unexamined Japanese Patent Publication No. 161900/1983 to eliminate the inconvenience of clear distinctive imaging of the border between the printed and unprinted regions. However, the highest compensation in speed or sensitivity accomplished by this prior proposal is only about 2 times, with attendant disadvantages that it is required to print for two or more times, and that various problems relating to color scheme or printing positions are involved therein.